1. Field of the Invention
The present invention is directed to motor vehicle wheel balancing systems, and more particularly to electronic measurement devices used on such systems for the purpose of entering wheel dimensions needed for determining sizes and locations of imbalance correction weights.
2. Description of the Related Art
It is well known in the automotive wheel balancing art that to compensate for a combination of static imbalance (where the heaviest part of the assembly will seek a position directly below the mounting shaft) and couple imbalance (where the assembly upon rotation causes torsional vibrations on the mounting shaft), at least two correction weights are required which are separated axially along the wheel surface, coincident with weight location "planes". For using clip-on weights the "left plane" comprises the left (innermost) rim lip circumference while the "right plane" comprises the right rim lip. If adhesive weights are used, the planes can reside anywhere between the rim lips, barring physical obstruction such as wheel spokes and welds. With the wheel assembly mounted to the balancer, the relative distances from a reference plane (usually the surface of the wheel mounting hub) to the planes are made known either by manually measuring with pull-out gauges and calipers and then entering the observed values through a keypad, potentiometer, or digital encoder, or by using an automatic electronic measuring apparatus. The radius at which the weights will be placed must also be entered, again either manually or by use of the electronic measuring apparatus. The balancer then determines what angular position the weights must be placed in the left and right weight planes and guides the operator through the display, typically a bar graph pertaining to each weight plane, to rotate the wheel assembly to a rotational position for each plane at which the correction weight is to be placed at the "12:00" position (straight up from the centerline of the wheel).
A problem sometimes arises with such systems when using adhesive weights when the operator enters the weight planes at less than optimum locations. The operator is usually unfamiliar with the importance of maximizing the distance between the two planes and is tempted to locate both near the inside of the wheel, much closer together than possible, for easier reach for weight placement. The closer the planes are together, the more weight is required to correct for the same couple imbalance, and the harder it is to balance below the "blind" (display for a weight plane shows zero if the residual imbalance is below the blind amount). The situation is further aggravated by the fact that adhesive weights are easily mis-applied since there is no physical means forcing the weights to reside on the correction planes as with rim-lip mounted weights. U.S. Pat. No. 4,891,981 discloses a system where an automatic measuring apparatus is used to input the weight planes and is later used again as a placement aid to insure the distances to the applied weights agree with the previously inputted planes. This is an improvement over the prior art as long as the planes are inputted at reasonable locations, but this often is not the case. It also has been observed that the operator is tempted to input the planes at grooves or score lines within wheels to use these features as a reference later to place weights. In the case of a groove, the adhesion of the weight to the wheel is hampered because of the depressed shape of the groove and because the groove allows road water to enter the underside of the weight. In the case of a score line, it is possible that the surface may have a taper which the operator did not notice. If a long strip of adhesive weights is required, a substantial portion of the weight will tilt away from the weight plane as it is pressed to conform to the conical surface formed by the taper, resulting in balancing error. Adhesive weight balancing is even avoided by many operators because of the aforementioned effects from expecting the operator to locate the weight planes. Quite simply, what is needed is to make adhesive weight balancing as easy as clip-on weight balancing.
A second problem with such systems exists because incremental weights are used. Using the most widely used increment of 0.25 oz., the required weight amount, no matter how accurately calculated and no matter to how fine a resolution, must be rounded to this increment. Residual imbalance as high as half the blind per weight plane will certainly occur. U.S. Pat. No. 4,891,981 to Scholdfield discloses a system which provides automatic iteration and comparison of different weight amount increments and angular positions for the two weight planes to minimize residual static imbalance, but it does so at the expense of residual dynamic imbalance. The rounding and blind problem can be overcome by shrewd users by disabling the blind and rounding and repeatedly re-entering the plane locations either manually or with an automatic input device (almost all balancers allow weight recalculation without having to re-spin the wheel assembly) until the displayed weights are very close to available incremental amounts. This process is still very time consuming and requires the operator to interpret the meaning of weights displayed to fine resolution. Furthermore, adjusting one plane location can affect the required amount and location of the weight required for the other plane, potentially confusing the unskilled operator. What is needed is to release the operator from the task of determining where the planes need to be, as well as a fast way to adjust plane locations and to know what the result of the balance job will be because of the adjustment.
A third problem with such systems is angular weight placement error by the operator. Present balancers precisely guide the operator to rotate the wheel to weight placement position to within as little as 0.35 degrees, corresponding to less than 1/16" movement at the lip of a 15" wheel. Unfortunately, the operator must judge for himself where the 12:00 position is for placing the weight. Placement errors of more than 1/2" are commonplace. The larger the weight, the harder it is to judge the 12:00 position, and the more residual imbalance a misplacement will cause. Working with the inside surface of a wheel to place adhesive weights is the most error prone situation of all since the operator must crouch down and look upside-down into the mounted wheel to see the 12:00 position. Light spotters of various types have been offered which project a line onto the wheel/tire assembly but these devices add cost, can only spot one side of the tire at a time, and/or cannot spot the inside of the wheel for adhesive weight placement. U.S. Pat. No. 5,447,064 to Drechsler discloses a novel electronic measuring apparatus geometry where a pivoting telescoping arm insures that the contact point of the device on the wheel is always at the 12:00 position. This design, however, makes cleaning of the wheel surface for adhesive weights difficult since the contact point is not visible to the operator. Also the design forces the apparatus to reside on top of the weight tray at the front of the balancer even when not in use, exposed to carelessly tossed mounting cones and taking up valuable space in the tray that could have been used for weight storage pockets.
A fourth problem with such systems is that even when the wheel assembly is angularly positioned for a weight placement, it can rotate away when the operator lets go if the static imbalance component overcomes the friction of the mounting shaft bearings and drive system. It is also difficult for the operator to keep from bumping into the assembly while leaning over the tire to judge the 12:00 position. Foot brake devices are offered by some manufacturers to address this problem but their use can make it difficult to reach for weights, instruction manuals, etc., while standing on the foot brake.
A fifth problem exists regarding the display of the data obtained by electronic measuring devices with such systems. Firstly, the operator cannot easily tell if the device is working correctly. The distance and diameter dimensions electronically measured for each weight plane are typically displayed with LED's or as values on a CRT display. Operators tend to trust the electronic measuring device, however, and ignore these displayed values. A measurement in error goes unnoticed and the resulting displayed balance weight amounts and placement angles are incorrect as well. For example: The distance entered for the left plane is usually expressed in mm. If the electronic measuring device is out of calibration and results in a left plane input display of 125 instead of a correct value of 135, the operator is unlikely to notice and will "chase weights", where each spin calls for more weight because the weights are not being placed where the balancer thinks they are. Verifying the accuracy of a display of digits for each wheel balance job would defeat the purpose of electronic dimension entry because some kind of mechanical distance and/or internal diameter gauge would have to be applied at each weight plane and compared to the display digits. Secondly, it is often confusing on such systems when switching between adhesive weight modes and clip-on modes. Because adhesive weight locations are measured at the inside surface of the wheel while clip-on weight locations are measured at the rim lips (nominal rim data), the measurement procedure is different for each type of weight (discounting methods where the adhesive weight locations are estimated based on nominal wheel data). Most systems allow taking data for one type without changing the other. If the user forgets this fact and switches to the other type of weight to balance the wheel, the weight values shown are calculated from data taken from the last wheel assembly which was used in that mode. One manufacturer requires entering adhesive locations as well as rim lip locations before calculating weights; however this requires double the effort and confuses the operator as to why both input procedures must be used when only one type of weight will be used. Some balancers have the capability of switching context between multiple operators where operator B, for example, can interrupt operator A by recalling his last wheel data without retaking of data and without destroying operator A's data. The prior art where the only item that changes is the wheel data is especially dangerous in this case as the operators probably do not remember the dimensions of their particular wheel assemblies. It has been observed that even though some kind of display prompt is given as a reminder as to which operator is currently enabled, operators can accidentally balance their wheels with the wrong operator mode, especially if they did not notice that another operator changed the mode.